Recently, wind power generation that uses clean and inexhaustible energy has attracted attention. Since a power generator body comprising a windmill is set at a height of several tens of meters in a case of a large wind power generation facility, a large amount of labor is needed and high risks are accompanied in maintaining the bearing that supports the main shaft of the blade of the windmill. Therefore, the bearing that supports the main shaft of the wind power generator requires high reliability and a long durable life.
A self-aligning roller bearing suitable for rotatably supporting the main shaft of the wind power generator has been disclosed in Japanese Unexamined Patent Publication No. 2004-11737, for example. As disclosed in this document, a large double row self-aligning roller bearing 1 shown in FIG. 1 is used as the bearing for supporting the main shaft in the large wind power generator in many cases.
As a main shaft 2 of a windmill of the wind power generator is mounted on a housing 4 so as to support the top end provided with a blade 3 in a cantilevered manner, the large self-aligning roller bearing 1 that can support the deflection of the main shaft 2 is used as a cantilever bearing. When the blade 3 receives wind force, the main shaft 2 rotates with the blade 3. This rotation of the main shaft 2 is speeded up by a speed-up gear (not shown) and transmitted to a power generator.
The self-aligning roller bearing 1 comprises an inner ring 5, and outer ring 6, and double row spherical rollers 7 and 8. While the power is generated by wind force, axial direction load (bearing thrust load) due to wind power applied to the blade 3 and a radial direction load (bearing radial load) due to self-weight of a blade shaft are applied to the main shaft 2 supporting the blade 3. Since the double row self-aligning roller bearing 1 can receive the radial load and the thrust load at the same time and has a self-aligning property, it can absorb the inclination of the main shaft 2 due to a precision error or a mounting error of the housing 4, and absorb the deflection of the main shaft 2 during operation.
The inner ring 5 shown in FIG. 1 has a center rib 9 abutting on end surfaces 7a and 8a of the spherical rollers in the right and left rows. When the end surfaces of the spherical rollers 7 and 8 have convex spherical configurations, in order to prevent the spherical rollers 7 and 8 from being skewed, both sides of the middle 9 have concave curved surfaces to be matched to the concave spherical configurations of the spherical rollers so that the contact area between both is increased in general.
According to the above double row self-aligning roller bearing 1 for supporting the main shaft of the wind power generator, the thrust load is higher than the radial load during the operation of the windmill. In this case, the spherical roller 8 in the row positioned farther from the blade 3 receives the radial load and the thrust load at the same time. As for the spherical roller 7 in the row positioned closer to the blade 3, the thrust load is not so much applied and only the radial load is applied to it.
Meanwhile, in a windless state, the load applied to the main shaft support bearing 1 is mainly the radial load. Therefore, the spherical roller 7 in the row positioned closer to the blade 3 receives a higher radial load in the windless state in which the windmill does not rotate, than the state in which the windmill rotates.
As described above, in the case of the double row self-aligning roller bearing 1 for supporting the main shaft of the wind power generator, since the spherical roller 8 in the row positioned farther from the blade 3 is highly loaded, its rolling fatigue life becomes short as compared with the spherical roller 7 in the row positioned closer to the blade 3. Especially, since the concave spherical configuration of the spherical roller 8 is in contact with the concave curved surface of the center rib 9 of the inner ring 5, the contact surface pressure is high, so that friction resistance is generated and a rotation torque is increased at the contact part. Furthermore, since the contact position is at the upper part of the side surface of the center rib, a contact ellipse is cut and an edge stress is generated at the upper end and as a result, this part could be worn at an early stage or peeled and the like.
Meanwhile, the spherical roller in the row positioned closer to the blade 3 is low-loaded, so that sliding is generated between the spherical roller 7, and track surfaces 5a and 6a of the inner ring and outer ring 5 and 6, causing surface damage and abrasion. Although it is considered to increase the size of the bearing in order to correspond to the high load, it is a waste for the spherical roller on the low-loaded side.